1. Field of the Invention
The present invention relates to a semiconductive rubber roller and more particularly to a semiconductive rubber roller, having a toner-transporting portion, which is used as a developing roller, a cleaning roller, a charging roller, and a transfer roller and the like mounted on an electrophotographic apparatus.
2. Description of the Related Art
In recent years, in the printing technique using the electrophotographic method, a high-speed printing operation, formation of a high-quality image, formation of a color image, and miniaturization of image-forming apparatuses have been progressively made and become widespread. Toner holds the key to these improvements. To satisfy the above-described demands, it is necessary to form finely divided toner particles, make the diameters of the toner particles uniform, and make the toner particles spherical. Regarding the technique of forming the finely divided toner particles, toner having a diameter not more than 10 μm and not more than 5 μm have been developed recently. Regarding the technique of making the toner spherical, toner having not less than 99% in its deviation from a spherical form has been developed. To form the high-quality image, polymerized toner has come to be widely used instead of pulverized toner conventionally used. The polymerized toner allows the reproduction of dots to be excellent in obtaining printed sheets from digital information and hence a high-quality printed sheet to be obtained.
In compliance with to the improvement in the technique of forming finely divided toner particles, making the diameters of the toner particles uniform, making the toner particles spherical, and the shift from the pulverized toner to the polymerized toner, in an image-forming apparatus of an electrophotographic apparatus such as a laser beam printer, and the like, a semiconductive rubber roller is useful as a developing roller which imparts a high electrostatic property to toner and is capable of efficiently transporting the toner to an electrophotographic photoreceptor without the adhesion of the toner to itself. Users demand that the high-performance function of the semiconductive rubber roller is maintained to the end of the life of a product, for example, the electrophotographic apparatus on which the semiconductive rubber roller is mounted.
To comply with such a demand, semiconductive rubber rollers composed of an ionic-conductive rubber are proposed. More specifically, the following semiconductive rubber rollers are proposed.
For example, as disclosed in Japanese Patent Application Laid-Open No. 2004-170845 (patent document 1), the present inventors proposed the conductive rubber roller composed of an ionic-conductive rubber, having a uniform electrical characteristic, which contains a dielectric loss tangent-adjusting filler to adjust the dielectric loss tangent thereof to 0.1 to 1.5.
The conductive rubber roller is capable of imparting a proper and high electrostatic property to toner, thereby providing a high-quality initial image. In the conductive rubber roller, the charged amount of the toner little decreases even after printing on a considerable number of sheets finishes. Consequently the conductive rubber roller keeps providing a high-quality image for a long time.
As disclosed in the patent document 1, a rubber component, represented by epichlorohydrin rubber, which contains chlorine atoms is used for the conductive rubber roller to allow the roller to be ionic-conductive. In this case, the rubber component containing the chlorine atoms has a high surface free energy. Thus the toner and an additive to be added thereto and the rubber component containing the chlorine atoms are liable to adhere to each other. When the rubber component containing the chlorine atoms is polymerized with an ionic-conductive ethylene oxide monomer, the conductive rubber roller has a large surface free energy and is liable to get wet, which is a main cause of the adhesion of the toner to the conductive rubber roller. When the oxide film is formed on the surface of the conductive rubber roller by irradiating the surface thereof with ultraviolet rays or exposing it to ozone, the oxygen concentration of the surface of the conductive rubber roller becomes high. Thus the surface free energy increases, which is also a main cause of the adhesion of the toner to the conductive rubber roller.
When the dielectric loss tangent of the conductive rubber roller is set to 0.1 to 1.5, the electrostatic property of the toner can be improved and hence the transport amount of the toner can be decreased. Thus the conductive rubber roller provides a high-quality image such as a half-tone image. On the other hand, the amount of the toner deposited on a developing roller decreases, which is also a main cause of the adhesion of the toner to the conductive rubber roller.
The toner which has adhered to the semiconductive rubber roller does not considerably affect images formed in an early stage and when images are successively printed. But when images are printed in the following conditions (1) through (4), the influence of the toner that has adhered to the semiconductive member cannot be ignored. For example, normally, charged toner is transported to an electrophotographic photoreceptor having an opposite electric charge by a static electricity (Coulomb force). But the transport of the toner by the static electricity is prevented because the adhesion of the toner to the developing roller is high. Thus there arises a problem that the print concentration becomes low, although the charged amount applied to the toner does not change.
(1) When printing is made on a considerable number of sheets of paper and hence toner has an affinity for a developing roller (for example, image is printed at 1% on about 2,000 sheets of paper).
(2) When an average particle diameter of toner is not more than 8 μm and particularly not more than 6 μm.
(3) When printing is made not successively, but is suspended and made the next day.
(4) When a printer is used in a low-temperature and low-humidity environment in which the charged amount of toner is comparatively large.
Proposed and disclosed in Japanese Patent Application Laid-Open No. 2005-225969 (patent document 2) is a semiconductive rubber member composed of the ionic-conductive rubber containing the rubber having the polyether bond. Wax is added to the ionic-conductive rubber to improve processability and prevent nonuniformity in molding and formation of a defective surface such as a cracked surface and decrease the surface free energy so that the additive for toner can be prevented from adhering to the semiconductive rubber member for a long time.
But even in the semiconductive rubber member, there may be a decrease in print concentration in the above-described conditions. Thus there is room for improvement in the prevention of the adhesiveness of the toner to the semiconductive rubber member.
In addition, there may be a slight degree of contamination on the toner and an electrophotographic photoreceptor owing to the presence of a component having a low molecular weight caused by bleed of wax or the like and owing to the revelation of the adhesiveness of the semiconductive rubber member in environment having a comparatively high temperature of about 50° C. Therefore when the semiconductive rubber member is used for a printer or the like demanded to provide a high-quality image, the kind of rubber or polymer which can be used for the semiconductive rubber member is limited.
Proposed and disclosed in Japanese Patent Application Laid-Open No. 2001-357735 (patent document 3) is a conductive member including not less than two function layers (for example, conductive elastic layer) formed on the conductive base material. In the conductive member, at least one function layer other than the outermost layer is coated with a specific treating agent.
Proposed and disclosed in Japanese Patent Application Laid-Open No. 2002-23482 (patent document 4) is a metal developing roller having a resin coating layer, containing the metal phthalocyanine compound, which is formed on its peripheral surface.
When the surface of the conductive member and the developing roller is coated with the coating agent as described in these patent documents, the electrical characteristic of the conductive member is changed owing to the thickness of the coating layer and the dispersibility of a filler contained in the coating agent. Thus the uniformity of the electrical characteristic and the reproducibility of designed values are damaged. Even though materials such as an ionic-conductive rubber, metal, and the like excellent in the uniformity of the electrical characteristic thereof are used as the material to be coated, the above-described problem cannot be solved. Further the cost for producing the conductive member and the developing roller is high.
Patent document 1: Japanese Patent Application Laid-Open No. 2004-170845
Patent document 2: Japanese Patent Application Laid-Open No. 2005-225969
Patent document 3: Japanese Patent Application Laid-Open No. 2001-357735
Patent document 4: Japanese Patent Application Laid-Open No. 2002-23482